I'm starting to understand aperture and shutter speed more (thanks to Understanding Exposure thanks everyone for recommending it) but I have a question. After setting the aperture and shutterspeed to get the correct exposure if I change move my lense to zoom in that doesn't mess with the aperture does it? I don't think it does, but I just wanted to double check... Thanks!
I have a Canon Digital Rebel xt

it will if you have a "kit " lens. as you zoom out the aperture will change.
i.e. if the lens is a 3.5-5.6 that will mean at the shorter focal lenth the widest fstop will be a 3.5 at the longest focal length it will chantge to 5.6

you can't over ride that feature.

just watch your LCD and move the lens and you will be able to see at which focal length the fstop starts to change.

If you don' t want this change to occur you have to buy a 2.8 lens that will maintain the same fstop with all focal lengths on the lens.

It might also be down to how you metered the subject initially. If you used spot metering it might not be so much of a problem than if you _____ metered. Somebody fill in that blank for me please - brain fart.

It changes. So if you get the correct exposure when your at 18mm, and you zoom in to 55mm, your picture will be underexposed because the aperture will stop down to 5.6, letting less light in.

If your at 55mm and set the exposure there, and then go down to 18mm, or anywhere in-between, you will be over exposed. The aperture will open up and let more light in.

However, if your aperture was set higher (f/5.6+) when you were down at 18mm, and you zoomed in, nothing would change. So if you were shooting a wide-angle landscape at f/22, and then you zoomed in, your exposure would be the same.